A ball screw having a screw shaft, a nut and balls rollably arranged therebetween is greatly used in various fields such as positioning mechanisms of machine tools and robots, feeding mechanisms, steering gears of automobiles and the like as the ball screw enables reduction of a coefficient of friction in rotating the screw shaft relative to the nut.
Recently, in order to increase allowable loads, there is developed a roller screw using rollers instead of the balls. The roller screw has a screw shaft having an outer peripheral surface with s spiral roller rolling part formed thereon, a nut having an inner peripheral surface with a spiral loaded roller rolling part formed thereon facing the roller rolling part of the screw shaft, and a plurality of rollers rollably arranged between the roller rolling part of the screw shaft and the loaded roller rolling part of the nut. As the nut rotates relative to the screw shaft, the rollers roll between the screw shaft and the nut. Once each roller has rolled up to one end of the loaded roller rolling part of the nut, the roller is lead into an unloaded roller return path provided in the nut. Then, after passing through the unloaded roller return path, the roller is returned to the other end of the loaded roller rolling part.
Balls can roll in all directions. Meanwhile, rollers can roll only in one direction. In order to move the rollers smoothly from the unloaded roller return path to a loaded roller rolling path between the roller rolling part of the screw shaft and the loaded roller rolling part of the nut, there is a need to match an attitude of each roller inside the unloaded roller return path with an attitude of the roller inside the loaded roller rolling path at a connection part of the loaded roller rolling path and the unloaded roller return path. To meet this need, as shown in FIGS. 18 and 19, a midpoint of an unloaded roller return path 51 is twisted so that an attitude of each roller 52 is changed when the roller 52 moves in the unloaded roller return path 51 (the central axis of the roller 52 is inclined) (see, for example, the patent document 1). Besides, the cross sectional shape of the unloaded roller return path 51 is formed to be a quadrangle similar to a cross sectional shape of the roller 52 taken along the central axis of the roller 52.
[Patent Document 1] Japanese Patent Laid-open Publication No. 11-210858